This invention relates generally to measurement instruments and, more particularly, to instruments for measuring the vertical tracking angle of phonograph pickups.
Shortly after the commercial introduction of the stereophonic audio disc record in 1957, record and pickup manufacturers became aware of the vertical tracking angle problem in record production. Among several papers describing the problem and techniques for measuring the vertical tracking angle are two by B. B. Bauer entitled "The Vertical Tracking Angle Problem In Stereophonic Record Production", IEE Trans. Audio, Volume AU-11, pp. 47-55 (1963 March-April) and "Vertical Tracking Improvement in Stereo Recording", Audio, pp 1-4 (February 1963). The vertical tracking angle is defined as the angle between two straight lines: one normal to the plane of the phonograph record and the other in the direction along which the stylus vibrates on the average while reproducing a purely vertical modulation on the record. To avoid vertical tracking distortion, this angle must equal the effective vertical angle with which the record grooves were recorded. As a result of the tests and conclusions presented in these and other papers the Recording Industry Association of America (RIAA) in 1961 recommended a vertical tracking angle of 15.degree. for stereophonic cutters in order to provide a closer match to the vertical angle of pickups. This would presumably result in a vertical angle on records which would be easier to achieve in pickups, thereby reducing vertical tracking angle distortion.
In spite of the early effort toward standardization, recent studies by applicants and their colleagues have indicated that there still remains a large discrepancy between the vertical angles at which records are cut and the angles at which the information is played back. Part of the reason has been the lack of a precise and easy-to-use technique for measuring the vertical tracking angle in phonograph pickups. With a view toward satisfying the long-felt need for a suitable instrument for measuring vertical tracking angle, applicants and their colleagues have recently concluded a study to experimentally determine the accuracy and repeatability of various measurement techniques. Three test record-based approaches were examined and found to separately yield repeatable vertical angle measurements and to give results which were in close agreement with each other. A mechanical technique was also tried and found to be consistent with the test record-based measurements, albeit with somewhat poor repeatability. The details of these experiments and conclusions drawn therefrom are described in two papers authored by James V. White and Arthur J. Gust entitled "Measurement Of FM Distortion In Phonographs", Journal of the Audio Engineering Society, Vol. 27, No. 3, pp 121-133 (March 1979) and "Three FM Methods For Measuring Tracking Angles of Phono Pickups", Journal Of The Audio Engineering Society, Vol. 27, No. 4, pp 242-249 (April 1979), the content of both of which is hereby incorporated herein by reference. The second of these papers suggests that one of the measurement techniques evaluated, characterized as the "real-sine-wave" method could, in principle, be embodied in hardware that would permit the vertical angle of a pickup to be read from a meter while a single band on a CBS STR-112 test record and being played. The present invention is directed to the design and construction of an instrument which extracts and directly displays on a meter (without off-line computation) the vertical tracking angle of a pickup during playback of a known test record signal.
As background to a better understanding of the nature of the tracking angle problem and how it is addressed by the present invention, and to establish the coordinate system and applicable mathematical formulae, the disclosure of the aforementioned White and Gust papers will be briefly reviewed. Detailed derivations of the applicable mathematical formulae are contained in the aforementioned White and Gust papers and/or in the literature referenced therein and will not be repeated here. FIG. 1 schematically depicts a typical disc cutter, which is suspended and pivoted above the record surface such that the cutter tip motion is contained in a plane slanted away from the point of suspension. The angle .theta..sub.R between a line vertical to the record surface and a line perpendicular to an imaginary line drawn between the pivot point of the cutter assembly and the cutter tip, is defined as the vertical recording angle. If, for example, a sinusoidal vertical signal (A) is applied to the cutter, the modulation actually cut in the record is really contained in an inclined coordinate system as shown in waveform (B). As may be seen, what started as a simple sinusoidal signal is now a signal which is considerably altered in shape. Such a signal will produce distortion upon playback unless the transformation is accurately corrected.
The vertical tracking angle .theta..sub.P of a pickup cartridge is shown in FIG. 2 which depicts an ideal pickup with a rigid stylus shank, the figure suggesting (in exaggerated form) that the dynamic pivot point of the stylus shank may not be located along the longitudinal axis of the shank when an audio signal is reproduced. As indicated, the vertical tracking angle is measured from a line vertical to the record surface to an imaginary line perpendicular to an imaginary line drawn through the average position of the dynamic pivot point and the tip of the pickup stylus. It has been established that to avoid vertical tracking angle distortion, the pickup should apply the inverse of the transform that occurred during the cutting process. This is readily achieved if the pickup vertical angle .theta..sub.P equals the recorded vertical angle .theta..sub.R.
In practice, in spite of the abovementioned RIAA standard, it has been found that the vertical angle of modern pickups does not match the vertical angle at which modern records have been cut. Vertical angles for three major disc cutters, used to produce most of the records made today, vary between 15.degree. and 22.degree.. The average vertical angle of ten high quality pickups that were measured in the course of the study was 29.degree., with angles of some as small as 22.degree. and of others as large as 33.degree.. Thus, a mismatch between the vertical angles .theta..sub.P and .theta..sub.R of as much as 18.degree. can be expected with these combinations of record and pickup. The distortion due to vertical tracking angle mismatch is of greater concern today than heretofore for several reasons:
1. Present records often contain greater vertical signal content because of unique spatial effects or phase interactions which occur during a multitrack mixdown.
2. Present records are cut at levels which are several dB greater on the average, thereby directly increasing the percentage of vertical angle distortion.
3. Distortion in other components of audio recording and playback systems has been greatly reduced so that vertical tracking-produced distortion is likely to be the major cause of the total distortion in the reproduction chain.
While many techniques for the measurement of vertical tracking angle have been described in the literature, several suffer the disadvantage that they cannot be performed rapidly; others require the use of complex signals on test records which are difficult to produce accurately or require several bands of a test record to be played and calculations to be made to achieve an answer; and still others are readily contaminated by other forms of distortions, such as tracing.
Most investigators who have addressed the problem of measuring vertical tracking angle have concluded that the most reliable electrical technique is one in which a pair of test signals are recorded on the record in such a manner that distortion will produce an FM modulation during playback. Of the known FM techniques for the measurement of the vertical tracking angle, the "real-sine-wave" method described by White and Gust, which utilizes a readily available CBS STR-112 test record, has been emobodied in the measurement device of the present invention because it offers the best combination of speed and accuracy.
Briefly reviewing the "real sine wave" method described by White and Gust, the test setup shown in block diagram form in FIG. 3 is used together with a CBS STR-112 test record, using Group 2B of the bands on the record, which has 400 Hz and 4 KHz signals recorded together vertically with an effective vertical recording angle .theta..sub.R =16.5.degree..+-.1.degree.. The appropriate channel signal (i.e., left or right) from a velocity-sensing pickup 10 is fed to a highpass filter 12 which passes frequencies above about 2 KHz to an FM discriminator 14. The output from discriminator 14 is a 400 Hz signal whose amplitude is proportional to the frequency deviation of the 4 KHz signal produced at the stylus tip. This signal is filtered by a bandpass filter 16, the output from which, designated E.sub.2, is compared to the 400 Hz signal recorded directly on the record; this signal, designated E.sub.1, is obtained by bandpass filtering, in filter 18, the signal generated by the velocity pickup 10. A phase meter 20 is provided to measure the phase lead of the 400 Hz frequency deviation of of the 4 KHz tone with respect to the recorded 400 Hz tone. It has been shown that tracking angle errors produce 400 Hz frequency deviations that are strictly either in phase or out-of-phase with the recorded 400 Hz in a velocity sensitive signal. In contrast, tracing errors produce frequency deviations that are strictly in quadrature. Thus, in the test setup of FIG. 3, if the 400 Hz voltage waveforms E.sub.1 and E.sub.2 are represented by phasors in the complex plane, and E.sub.1 is defined to be real and positive, then only the real part of E.sub.2 is influenced by the tracking angle error, in a properly calibrated experiment. Thus, the vertical tracking angle .theta..sub.P of the pickup at 400 Hz is given by the following formula when the real FM deviation is caused by tracking-angle errors: EQU .theta..sub.P =tan.sup.-1 [tan .theta..sub.R .+-.9.1.times.10.sup.-4 RF Cos .theta./V.sub.400 ]
where .theta..sub.P is the vertical playback angle in degrees; .phi..sub.R is the vertical recording angle; R is the groove radius in meters; V.sub.400 is the peak velocity in meters per second of the recorded 400 Hz tone; F is the peak 400 Hz frequency deviation of the 4 KHz tone in Hz; and .phi. is the phase lead of E.sub.2 with respect to E.sub.1 in degrees. The peak value of E.sub.2, read by voltmeter 22, is equal to kF, where k is the calibration constant of FM discriminator 14 in volts/Hz. The minus sign in the equation is used for data obtained from the left channel of the pickup output, and the plus sign for right-channel data. The correct polarity is usually obtained by making a known angular change (e.g., an increase) and determining whether the deviation follows suit (i.e., also increases).
The results published by White and Gust show that the test setup of FIG. 3 yields reliable measurements of the vertical tracking angle; however, the technique requires a considerable amount of expensive test equipment and has the significant disadvantage, from a user's standpoint, that it requires computation of the final results.
Accordingly, it is the object of the present invention to eliminate these restrictions and shortcomings by providing an electronic meter which allows direct reading of the vertical tracking angle.